Many mobile machines include a propulsion system with a multiple-ratio transmission operable to transmit power from a prime mover (such as an engine) to propulsion devices (such as wheels) at any of a plurality of optional drive ratios. Some multiple-ratio transmissions have a step-change configuration, meaning that the transmission has a finite set of discrete drive ratios at which it can transmit power. Other multiple-ratio transmissions, known as continuously variable transmissions, have a configuration allowing adjustment of the transmission's drive ratio through a continuous range. The advantages associated with continuously variable transmissions include the decoupling of the transmission input speed from the transmission output speed and the ability to rapidly adjust the drive ratio, the output speed, and the amount of torque output by the transmission. This may help the propulsion system meet operator requests for abrupt changes in the travel speed of the mobile machine. Unfortunately, the rapid adjustment capability of continuously variable transmissions creates the possibility of undesirably abrupt adjustment in response to operator inputs.
U.S. Pat. No. 5,931,884 to Ochiai (“the '884 patent”) discloses a method of controlling a continuously variable transmission, including calculating an upper limit for the rate at which the drive ratio of the continuously variable transmission changes. The '884 patent expresses concerns that some control strategies for continuously variable transmissions may produce undesirable deceleration in particular circumstances by changing the drive ratio of the continuously variable transmission too rapidly and thereby causing negative torque at the output shaft of the continuously variable transmission. In order to address this concern, the control method of the '884 patent involves calculating the maximum rate at which the drive ratio of the continuously variable transmission can change without causing its output torque to drop to or below zero. The control method of the '884 patent imposes this calculated maximum rate as a limit on the adjustment of the drive ratio of the continuously variable transmission, thereby preventing deceleration due to rapid adjustment of the continuously variable transmission.
Although the control method of the '884 patent involves calculating and imposing a limit on the rate of change of the drive ratio of the continuously variable transmission, certain disadvantages persist. For example, the adjustment limit imposed by the control method of the '884 patent limits acceleration in the same manner in all circumstances. Thus, the '884 patent fails to recognize that the desirable limits on acceleration may vary depending on various operating conditions.
The propulsion system and control methods of the present disclosure solve one or more of the problems set forth above.